fix document mistake (#3304)

message.getBody() return byte[], so change to length.

docs/en/Example_Batch.md

@@ -57,7 +57,7 @@ public class ListSplitter implements Iterator<List<Message>> {
         return currIndex; 
     }
     private int calcMessageSize(Message message) {
-        int tmpSize = message.getTopic().length() + message.getBody().length(); 
+        int tmpSize = message.getTopic().length() + message.getBody().length; 
         Map<String, String> properties = message.getProperties();
         for (Map.Entry<String, String> entry : properties.entrySet()) {
             tmpSize += entry.getKey().length() + entry.getValue().length(); 
@@ -78,4 +78,4 @@ while (splitter.hasNext()) {
        // handle the error
    }
 }
-```
\ No newline at end of file
+```